The antagonistic relationship between chlorophyll a concentrations and the growth areas of Trapa during summer in a shallow eutrophic lake

The relationship between concentrations of chlorophyll a in an open water area and growing areas of the macrophytes Trapa spp. and Nelumbo nucifera was investigated to clarify the role of macrophytes in phytoplankton growth in summer in a eutrophic shallow sand lake, Lake Honsagata, Japan. The chlorophyll a concentrations formed peaks in June 2007 and July 2008 in summer with cyanobacterial blooms composed of mainly Microcystis, Anabaena, Phormidium, and Oscillatoria, which decreased toward August and were maintained at different annual levels. The decline of blooms in August was caused by the rapid growth of macrophytes. The composition of phytoplankton and the level of bloom development in summertime differed characteristically from year to year. Also the total vegetation area of N. nucifera and Trapa spp. showed annually marked changes. A significant negative correlation between the concentrations of chlorophyll a and the growth areas of Trapa spp. in August was detected, indicating that the floating-leaved plants, Trapa spp., produced irregular, clear and turbid water states in this shallow eutrophicated lake. These antagonistic relations are explained based on a likely scenario of allelopathic effects on the development of cyanobacteria by Trapa spp. vegetation and the nutrient absorption competition among them. Our study demonstrated the potential of Trapa spp. to control cyanobacterial blooms producing harmful toxins.